An UV hardening type ink-jet image forming method is noticed as a method capable of solving the problem on the usual ink-jet method since the UV hardening type ink-jet method has merits such as that any specific substrate is not needed, the image formed by the method is excellent in the light fastness and the method is excellent in the working safety because the ink contains no solvent. The UV hardening type ink-jet method has high suitability to printer with high writing speed such as a line printer since drying process is not needed and the hardening time is short.
The UV-hardenable ink-jet ink is different from the usual printing ink and a coating material since such the ink is required to have low viscosity before the hardening so that the ink can be ejected from an ink-jet head. Thus, any reactive resin or binder almost cannot be added. Consequently, high polymerization degree is necessary. Accordingly, the hardening property and the property of the hardened layer tend to be varied depending on exterior factors compared with the printing ink and the coating material and the stable image formation is difficultly performed.
A high out put UV irradiation lamp such as a high-pressure mercury lamp and a metal halide lamp is usually used for hardening the ink image. Such the lamp causes occurrence of ozone since the lamp irradiates short wavelength UV light of not more than 200 nm. Therefore, there is a case that it is necessary to install an exhauster so as to prevent the pollution of the working environment since ozone is a gas harmful for the working environment. Further, in the case of the ink-jet ink, it has been found by the inventors that ozone causes a bad influence such as that the hardening ability of the image surface is degraded and the durability of the image is lowered even when the concentration of ozone is relatively low so that the influence on the working environment is a little. Such influences of ozone are not recognized until now.